Cardiac fibrillation is a potentially life-threatening emergency medical condition in which the heart muscle quivers instead of contracting in a coordinated fashion. An implantable cardioverter defibrillator (ICD) is a medical device that is implanted in a patient for the purpose of automatically detecting and arresting fibrillation. The ICD restores a normal heart rhythm without requiring immediate medical intervention using an external defibrillator (e.g., electric shock “paddles”). When an ICD detects atrial or ventricular fibrillation, coordinated muscular contractions are restored by internal delivery of a therapeutic electric shock to the heart. ICD hardware includes electronics contained in a biocompatible, hermetically-sealed housing that is implanted subcutaneously in the patient's chest; and electrodes, connected to the device by leads that extend into the heart, for sensing electrical signals and for applying electric current to the heart tissue. The ICD is configured such that one electrode is placed in either the atrium or the ventricle, and the ICD housing is electrically grounded. In the same manner that an external defibrillator effects a therapeutic shock, a battery within the ICD housing supplies power to charge a capacitor, which is then suddenly discharged across the heart. Ventricular defibrillation typically involves application of higher voltages, above about 240 V, associated with an energy level of about 40 J. Atrial defibrillation, however, typically involves application of lower voltages, having a maximum value in the range of about 200-240 V, and an associated energy in the range of about 4-12 J. Lower energy is required because of the reduced mass of cardiac tissue in the atrium, compared with that in the ventricle.
A conventional ICD applies a truncated, decreasing exponential voltage waveform that terminates fibrillation at a defibrillation threshold level (DFT), or energy requirement, of about 20-30 Joules. It has been shown by R. A. Malkin, et al., IEEE Transactions on Biomedical Engineering, 53:1492-8, 2006, that the decreasing truncated exponential waveform is not the most efficient for defibrillation. Studies such as those presented by S. R. Shorofsky, et al., Heart Rhythm, 2:388-94, 2005, have shown that an increasing exponential waveform shape is the most energy efficient at a given DFT. For example, M. W. Kroll, Pacing and Clinical Electrophysiology (PACE) 17:1782-92, 1994, and M. G. Fishler, IEEE Transactions on Biomedical Engineering, 47:59-67, 2000, disclose the use of specialized waveforms, instead of the usual decreasing exponential waveform, that are capable of reducing the DFT by 20% or more, resulting in a longer battery life, increased efficacy, or reduced device size. It has also been shown by G. Boriani, et al., Journal of Cardiovascular Electrophysiology, 18:728-734, 2007, that a square waveform produces the least amount of pain for the patient. Pain reduction is particularly important in cases of atrial fibrillation, in which the patient is likely to be conscious while therapeutic shocks are administered.
Although the most desirable waveform shapes are known, the ability to make use of the known optimum shapes is not commercially available in ICD products due to power constraints. For example, U.S. Pat. No. 7,450,995, to the same inventor as the present patent application, discloses the use of a specialized, rising exponential waveform, which has been shown to outperform the conventional truncated, decreasing exponential waveform. However, the '995 patent fails to present a practical implementation that has a high enough efficiency to benefit from use of the specialized waveform. Fischler, IEEE Transactions on Biomedical Engineering, January, 2000, demonstrates that in order to realize the benefit of using an increasing exponential waveform, the efficiency of the waveform generation circuitry must exceed 66%. If the energy needed to create a specialized waveform is greater than the amount by which use of that specialized waveform decreases the DFT, then no energy savings are realized. For example, if the output stage is 50% efficient, but it only decreases the DFT by 20%, then 30% of the energy is wasted. Analysis of circuitry described in the '995 patent has demonstrated that, because it is resistor-capacitor based, its efficiency is approximately 50%, which therefore is not implementable in an implantable device. U.S. Pat. No. 7,151,963 to Havel, et al. teaches an implementation that may be capable of achieving a significantly higher efficiency. However, it appears that the use of isolated switching circuitry renders Havel's implementation too large for practical use in an implantable device. What is needed, therefore, is an implementation having switching circuitry that offers a high enough efficiency to accommodate practical use of the more desirable rising exponential waveform.